Limit valve



J. A. PALMER Jan. 26, 1960 LIMIT VALVE 2 Sheets-Sheet 1 Filed Feb. 23,1955 PRIMARY DIRECTION/9L CONTROL VALVE 1 r-E 4 A A Jan. 26, 1960 J,PALMER 2,922,439

LIMIT VALVE Filed Feb. 23, 1955 2 Sheets-Sheet 2 IN I/E/VTO R JOHN a.PAL HER,

LIMIT VALVE John A. Palmer, St. Louis, Mo., assignor to The EmersonElectric Manufacturing Company, St. Louis, Mo, a corporation of MissouriApplication February 23, 1955, Serial No. 489,935 19 Claims. (Cl.137-622) The present invention relates generally to valves, and moreparticularly to a fluid control valve for automatically stopping andretaining a fluid powered device in a predetermined position. v

The valve of the present invention is particularly useful in connectionwith fluid operated cylinders or rams where it is desirable that thereciprocating element be accurately positioned, without shock, at apredetermined end point in its stroke. Normally, the valve will beconnected between the fluid operated cylinder and the usual primarydirectional control valve associated therewith. The limit valve has adisplaceable fluid controlling element which is mechanically shifted asthe reciprocating element of the ram approaches a predeterminedposition, the arrangement being such that the fluid flow isprogressively diminished until the reciprocating element is brought to ahalt. If conditions should by any chance be such that the reciprocatingelement overshoots or drifts past the predetermined position, the fluidflow will be reversed through the cylinder so as to return thereciprocating element to the predetermined position and thus retain itthere. Notwithstanding its own ability to reverse the fluid flow to andfrom the cylinder, the limit valve does not at any time precludeimmediate response to a reversal of the primary directional controlvalve associated therewith.

It is an object of the present invention to provide a limit valve forfluid operated reciprocating devices which functions automatically so asprogressively to decelerate a reciprocating element and ultimately tostop it in a predetermined position.

It is another object of the invention to provide a limit valve for usewith a fluid operated reciprocator which functions to retain thereciprocable element of the reciprocator in a predetermined position atone end of its stroke.

,It is another object of the invention to provide a limit valve for usewith a fluid operated reciprocating device which functions automaticallyto provide directional control of movement of the reciprocating elementof the device so as to compensate for tendency of the reciprocableelement to be displaced in either direction from a predeterminedposition.

It is another object of the invention to provide a limit valve for usewith a fluid operated device and a primary directional control valvewhich avoids any delay in response of the device to manipulation of theprimary directional control valve.

The foregoing, along with additional objects and advantages, will beapparent from the following description taken with the accompanyingdrawings, in which:

Figure l is a schematic diagram showing the limit valve of the presentinvention installed for positional control of a fluid operated cylinder;

Figure 2 is a cross-sectional view of a dual valve conforming to theteachings of the present invention, the movable parts of the valve beingillustrated in normal rest position;

Figure 3 is a view similar to Figure 2, but drawn to reduced scale andshowing a valve spool in displaced position; and

Figure 4 is a view similar to Figure 3, but showing the valve spool in aposition of further displacement and showing also two ball check valvesin displaced position.

Referring to the drawings more particularly by means of referencenumerals, the numeral 10 designates alimit valve which is constructed inaccordance with the teachings of the present invention. Directingattention to Figure 2 of the drawings, it will be noted that theillustrated valve 10 is actually a dual valve. In other words, the valve10 could be separated along its horizontal center line, designated inFigure 2 by the numeral 12, to provide two identical limit valves, eachof which, incidentally, would conform to the teachings of the presentinvention. This being true, a description of the upper half of the valve10 will suflice for the lower half as well, and it will be understood,therefore, that each of the primed reference numerals designates anelement in the lower half of the valve 10 which finds its counterpart inthe upper half thereof. A valve body 14 is, of course, common to boththe upper and lower halves.

Considering now the upper half of the valve 10 as illustrated in Figurethe body 14 is provided with a main valve chamber 16 for accommodationof a reciproca-ble valve spool 18. While the chamber 16 general- 1ytakes the form of a cylindrical bore 20, it is at the same time providedwith a series of annular recesses spaced and sized as clearly indicatedin the drawings. Each of the annular recesses in turn communicates withone or more fluid passages formed in the body 14. Thus, working from theleft in Figure 2, an annular recess 22 communicates with a fluid passage24 ported to the outside of the body 14 through a tubular nipple 26. Anannular recess 28 communicates with a passage 30 which, if the upperhalf of the valve 10 constituted a single valve, would also be ported tothe outside. In the illustrated dual valve 10, however, the passage 30is continued in the form of an extension to be described hereinafter.

Besides communicating with the passage 30, the annular recess 28communicates also with a passage 32 which, in turn, communicates with abranch passage 34 connected into the aforementioned passage 24. Thepassage 32 includes an enlarged chamber 36 provided with an annular ballseat 38 at the end nearer to the recess 28 and containing a ball valve40 biased toward the seat 38 by a spring 42.

An annular recess 44 communicates with a passage 46 connected into apassage 48. Like the passage 30, the passage 48 would be ported to theoutside if the upper half of the valve 10 constituted a single valve. Ittoo, however, is, in the illustrated device, extended to form a passageto be described. The passage 46 includes an enlarged chamber 50 providedwith an annular ball seat 52 at the end farther from the recess 44 andcontaining a ball valve 54 biased toward the valve seat 52 by a spring56.

An annular recess 58 communicates with the previously mentioned passage34. The passage 34 includes an enlarged chamber 6%) provided with anannular ball seat 62 at the end nearer to the recess 58 and containing aball valve 64 biased toward the seat 62 by a spring 66.

An annular recess 68 communicates with a passage 70 connected into apassage 72 ported to the outside of the valve to through a tubularnipple 74.

An annular recess 76 communicates with the previously described passage48, and also with a passage 78 connected into the passage 70. Thepassage 78 includes an enlarged chamber 80 provided with an annularvalve seat 82 at the end nearer to the recess 76 and containing a ballvalve 84 biased toward the seat 82 by a'spring 86.

Finally, an annular recess 88 communicates with the previously describedpassage 72.

The cylindrical bore 28, as distinguished from the annular recessesabove enumerated, slidably receives the valve spool 18. 'Morespecifically, the bore 28 is adapted for snug slidable engagement withlongitudinally spaced portions of the spool 18, as will appear. Thus,the spool 18 includes portions 90, 92, 94 and 96 spaced from each otheras clearly illustrated in Figure 2. It is to be understood that theindividual lengths, as well as the spacing of these portions of maximumdiameter of the spool 18 bears a particular relation to the spacedarrangement of the aforementioned annular recesses in the body 14. Thisrelationship is also evident from Figure 2 and its significance willappear from a description of the operation of the valve hereinafter.

Considering the valve spool 18 further, it will be observed that theaforementioned portions 90, 92, 94 and 96 are interconnectedsuccessively by portions of reduced diameter designated as 98, 108 and102. There is also a portion 104 and a portion 106, each of reduceddiameter, at the outer ends of the portions 90 and 96, respectively.Finally, there is an actuating stem 108 and a guide stem 110 of stillfurther reduced diameter at opposite ends of the spool 18. The actuatingstern 108 extends through and beyond a cap 112 secured to the outersurface of the body 14 by means of screws 114. The cap 112 is providedwith a central boss 116 which serves, not only as a centering device forthe cap 112, but also as a limiting stop for abutment with the portion104 of the valve spool 18.

The guide stem 110 extends through a cap 118 secured to another externalface of the body 14 by screws 120. Thecap 118 has a boss portion 122which, along with an oppositely extending portion 124 is counterbored soas to provide a spring chamber 126 for accommodation of a compressionspring 128 serving to bias the valve spool 18 toward the aforementionedabutment with the boss 116 of the cap 112. It will be noted that theboss 122 of the cap 118 serves as a limiting stop for move ment of thespool 18 in the opposite direction, being engageable with the lateralface of the portion 96, as is clear from Figure 2.

Preferably, both the cap 112 and the cap 118 are provided withconventional sealing devices 130 and 132 for prevention of leakage atthe faces of the body 14 and along the valve stems, respectively.

Directing attention now to Figure 1, the schematic representation theredepicted shows a typical installation of the dual limit valve 18. Aspreviously indicated, the valve 10 is incorporated between a directionalcontrol valve assembly 148 and a fluid operated cylinder assembly 142.The directional control valve assembly 140 is a conventional deviceprovided with a control lever 144 which is effective to connect a fluidpressure line 146 with either the fluid line 74 or the fluid line 74,the lines 74 and 74 being connected into the valve as previouslydescribed. The lever 144 is also effective to connect a fluid returnline 148 with either of the fluid lines 74 or 74, and it will beunderstood that with the lever 144 in the illustrated position to theright, the pressure line 146 is connected to the line 74, while thereturn line 148 is connected to the line 74'. Movement of the lever 144to the left, however, would be effective to reverse these connections.

The fluid operated cylinder assembly 142 is a double acting devicehaving a reciprocating element 150 movable in either direction relativeto a stationary cylinder 152. The element 158 is effective to performuseful work, and may be attached to any suitable device including adevice which may impose a constant force tending to move the element158. Such an arrangement is shown schematically in Figure l in theattachment of a weight W to the element 150 by means of a cable 154passing over a pulley 156.

A collar assembly 158 having an inclined cam face 160 is adjustablysecured to a portion of the reciprocating element 158 as illustrated inFigure 1. Thus mounted, the face 160 is positioned to engage a roller162 mounted at the free end of an arm 164 of a bell crank assembly 166pivotally mounted to a stationary bracket 168. An arm 170 of the bellcrank assembly 166 has its free end provided with an adjustable screw172 and retaining nut 174 for abutting engagement with the end of theactuating stem 108 of the aforementioned valve spool 18.

As is clear from Figure l, the depicted arrangement shows a secondcollar on the reciprocating element 150 and a second bell crank adaptedfor operative engagement with the actuating stem 108 of the valve spool18. Inasmuch as these elements are similar in all except dimensionalrespects to those previously described, the elements of the second sethave been designated by the primed numerals of their counterparts.

The tubular line 26, previously described as connected into the passage24 of the valve 10, is connected at its other end into one end of thefluid cylinder 152. Similarly, the line 26 is connected from the passage24 in the valve 10 to the other end of the cylinder 152. Thus, it willbe understood that the establishment of fluid pressure in the line 26,along with a reduction of pressure in the line 26 will cause thereciprocating element to move to the right, whereas the establishment offluid pressure in the line 26, along with a reduction of pressure in theline 26, will cause the reciprocating element 150 to move to the left.

Operation Directing attention once more to Figure 2, the valve spools 18and 18' are shown in their normal rest positions, which is to say thatneither is operatively engaged by the bell cranks 166 or 166' describedabove. Considering once more only the upper half of the valve 10, it isapparent that with the spool 18 in the position mentioned the passage 72is communicated through the annular recess 88, through that portion ofthe bore 20 which joins the recesses 88 and '76, and through the latterrecess to the passage 48. In similar manner, the passage 24 communicatesthrough the annular recess 22, through that portion of the bore 20 whichinterconnects the recesses 22 and 28, and through the latter recess tothe passage 30. Clearly, the recited intercommunication between thepassages 72 and 48 and that between the passages 24 and 38 results, notonly from the position of the spool 18, but from the illustratedrelationship between the spacing of the annular recesses 22, 28, 76 and88 combined with the illustrated relative size and spacing of theportions 90, 92, 94 and 96 of the spool 18.

It will be obvious, of course, that intercommunication between passagesin the lower half of the valve 10 will correspond to that abovedescribed when the valve spool 18 is in its illustrated position ofnormal rest. It will also be obvious that with both the valve spool 18and the valve spool 18' in normal rest position, the passages 72 and24', which is to say the lines 74 and 26', are intercornmunicated, asare also the passages 24 and 72, or the lines 26 and 74'. Finally then,it is apparent that with the valve spool 18' in normal rest position,the common passages 48 and 38 have open communication with the lines 26and 74, respectively, and, similarly, with the valve spool 18 in normalrest position, the common passages 48' and 30' are in open communicationwith the lines 26 and '74, respectively. From the illustration of Figure1, it will be observed that one or the other of the spools 18 and 18 isalways in normal rest position, and that, at times, both may be.

Referring to Figure 1 and assuming now that the illustrated position ofthe lever 144 connects the line 74 with the fluid pressure line 146 andthe line 74' with the fluid return. line 148, and. assuming further thatthe reciproeating element 150 of the assembly 142 is initially in aposition somewhat to the left of that illustrated so that both the valvespool 18 and the valve spool 18' are in normal rest positions, fluidpressure will be communicated through the valve to the line 26, andthence to the left end of the cylinder 152. At the same time, the rightend of the cylinder 152 will be connected through the line 26, andthrough the valve 10 to the line 74'. Under these conditions, thereciprocating element 150 will obviously be moved to the right so thateventually the face 160 of the collar assembly 158 will engage theroller 162 of the bell crank assembly 166. The resulting pivotalmovement of the bell crank 166 will be reflected in displacement of thevalve spool 18 against the action of the biasing spring 128. As thereciprocating element 150 proceeds further to the right, the spool 18will be simultaneously progressively displaced until it approaches theposition illustrated in Figure 3. In the latter figure, it will beobserved that the portion 94 of the spool 18 now engages that portion ofthe bore 20 which separates the annular recesses 76 and 88, therebyobstructing direct communication between these recesses and stopping theflow of fluid under pressure from the line 74 to the line 26. Also, theportion 90 of the spool 18 has now engaged that portion of the bore 20which separates the annular recesses 22 and 28, thereby closing offcommunication between the lines 26 and 74 through the annular recess 22and stopping the return flow of fluid from the line 26 to the line 74'.In neither case, however, is communication between these linescompletely eliminated, inasmuch as one-way reversed flow past the ballvalves and 84 is still possible. Thus, notwithstanding the obstructionof fluid flow which tends to advance the element 150 to the right, acondition is avoided wherein both ends of the cylinder 152 would besealed ofl without the possibility of reversing the fluid flow so as toeflect reversed movement of the reciprocating element 150.

In its approach to the position of Figure 3, the portion 94 of the spool18 gradually constricts the area communicating the annular recess 76 andthe adjacent portion of the bore 20 which extends to the recess 88 sothat the flow of fluid under pressure from the line 74 through the valve10 and the line 26' to the cylinder 152 is progressively diminished. Atthe same time, the portion 90 of the spool 18 progressively constricts,and eventually obstructs, the area communicating the annular recess 22and the adjacent portion of the bore 20 which extends to the recess 28.As a result of this diminution of flow, the reciprocating element 150 isbrought to a gradual, although conceivably a very rapid, halt. Clearly,the relative rapidity with which the reciprocating element 150 is haltedis a function of its speed of move ment combined with the angle ofinclination of the cam face 160. p

In the event that a work load, such as indicated in Figure l by theweight W, should act upon the reciprocating element 150 so as to bias itto the left, it is obvious that any leftward movement of the element 150will be accompanied by a corresponding leftward movement of the valvespool 18 such that the fluid pressure connection will be reestablishedto the left end of the cylinder 152 to return both the element 150 andthe valve spool 18 to mutually balanced condition. If, on the otherhand, conditions should arise, either due to loading of the element 158or to leakage in the fluid flow system, wherein the reciprocatingelement 158 should be inadvertently moved to the right farther than theabove described position of equilibrium, the valve spool 18 would alsobe moved farther to the right and would occupy a position of furtherdisplacement as illustrated in Figure 4. In this figure, it will beobserved that the spool 18 has moved to a position wherein the portion94 has uncovered the annular recess 68 so that the latter is nowcommunicated through the bore 20 with the annular recess 58.

Now, inasmuch as the annular recess '68 is connected by means of thepassages 70 and 72 with the line 74, the fluid pressure from the latteris delivered against the ball valve 64 so as to unseat it and allow thefluid pressure to be communicated on through the passages 34 and 24 tothe line 26, and thence to the right hand end of the cylinder 152. Atthe same time, it will be noted that the rightward movement of theportion 92 of the spool 18 has partially uncovered the annular recess 44so as to communicate the same with the annular recess 28 and, hence,with the line 74'. In other words, not only is the line 74, containingfluid pressure, now connected with the line 26 extending to the righthand end of the cylinder 152, but also the left end of the cylinder 152is connected by way of the line 26, the passage 24, theintercommunicated recesses 22' and 28', the passages 30 and 46, theintercommunicated recesses 44 and 28, the passages 30 and 48', theintercommunicated recesses 76' and 88', and the passage 72 to the line74', which is connected for return fluid flow. Obviously, then, thereciprocating element is caused to return to the equilibrium positionillustrated in Figure 3.

It will be observed that the return fluid flow just traced is in adirection enabling the ball valve 54 incorporated in the passage 46 tobe readily displaced so as to accommodate the described flow. Althoughthe return fluid also engages the ball valve 40, this element ismaintained on its seat 38 by the pressurized fluid which occupies thechamber 36 at this time by virtue of the open communication of thelatter with the passage 34. If, however, the lever 144 be reversed whilethe valve 10 is in the condition of Figure 4, the resulting flow ofpressurized fluid through the line 74 will clearly act to unseat theball valve 46 and to seat both the ball valve 54 and the ball valve 64.It is aided in seating the balls 54 and 64, of course, by the fact thatthe pressure in the line 74 is now reduced for return fluid flow, thisreduced pressure being communicated to the ball 64 through the passages72 and 70, the intercommunicated recesses 68 and 58, and the passage 34,and to the ball 54 through the passages 72, 70, and 78, the recess 76,and the passages 48 and '46. Thus, until the valve spool 18 once moreapproaches its normal rest position, pressurized fluid will flowupwardly through the valve chamber 36 and return fluid will flowupwardly through the valve chamber 80. r

As soon as the reciprocating element 150 has advanced far enough to theleft to enable the spool 18 to return to its normal rest position, andconsidering still that the lever 144 has been moved to its left handposition, it is obvious that with the line 74' being now connected withthe fluid pressure line 146 and the line 74 being now connected with thefluid return line 148, the action of the valve 10 and the cylinderassembly 142 will only be reversed from that previously described. Inother words, the reciprocating element 150 *will continue to move to theleft until the valve spool 18 is actuated, the spool 18 meanwhileremaining in normal rest position.

Clearly, there has been provided a limit valve and an arrangementincorporating the same which together fulfill the objects and advantagessought therefor.

It is to be understood that the foregoing description and theaccompanying drawings have been given only by way of illustration andexample. It is further to be understood that changes in the fonn of theelements, rearrangement of parts, or the substitution of equivalentelements, all of which will be obvious to those skilled in the art, isviewed as being within the scope of the present invention, which islimited only by the claims which follow.

What is claimed is:

l. A limit valve comprising, in combination, a body provided with anelongated generally cylindrical valve chamber and a plurality of fluidpassages communicating therewith, at'least four of said passages beingported to the outside of said body, a valve spool movably disposed insaid valve chamber for selectively interconnecting said fluid passagesin different predetermined combinations, and at least three check valvesdisposed in selected fluid passages for controlling fluid flowtherethrough, each of said check valves being disposed between saidchamber and a respectively difierent outside port.

2. The limit valve of claim 1 wherein a first and a second portedpassage are communicated at a minimum of three longitudinally spacedpoints each with the main valve chamber, a third ported passage iscommunicated at a. minimum of two longitudinally spaced points with themain valve chamber, and a fourth ported passage is communicated at aminimum of one point with the main valve chamber.

3. The limit valve of claim 2 wherein at least one path of communicationof each of the first, second, and third ported passages with the mainvalve chamber is provided with a directional check valve.

4. The limit valve of claim 3 wherein at least two of the paths ofcommunication of the first ported passage are provided with directionalcheck valves.

5. The limit valve of claim 4 wherein the fourth ported passage and oneof the checked paths communicated with the first ported passage arecommunicated with the main valve chamber at substantially the samelongitudinal point, and wherein the unchecked path communicated with thethird ported passage and the checked path communicated with the secondported passage are communicated with the main valve chamber atsubstantially the same longitudinal point.

i 6. The limit valve of claim 5 wherein at least the respective passageswhich communicate with the main valve chamber at substantially the samepoints are permanently communicated with each other.

7. A fluid operated power assembly comprising, in combination, a fluidmotor including a reciprocable power transmitting element, a fluidcontrol valve connected to said fluid motor for selectively controllingthe reciprocations of said reciprocable element, valve meansinterconnected between said fluid control valve and said fluid motor forlimiting the extent of movement of said reciprocable element, said valvemeans comprising means defining a plurality of fluid passages thereinand a displaceable element adapted to obstruct fluid flow through saidpassages and thereby effect stoppage of the reciprocable element at apredetermined point, and means for interconnecting said displaceableelement with said reciprocable element for corresponding movementtherebetween over at least a portion of the range of movement of thereciprocable element.

8. The combination of claim 7 wherein the valve means is provided with amain valve chamber into which each of the fluid passages iscommunicated, the displaceable element being movable in said chamber toa displaced position whereby to obstruct fluid flow therethrough in atleast one direction.

9. The assembly of claim 8 wherein the reciprocable element is movablebeyond the aforesaid predetermined point, such movement being efiectivethrough the interconnecting means to move the displaceable elementbeyond an intermediate position for obstructing fluid flow to a positionfor diverting fluid flow in the fluid passages and thereby efiect areverse movement of the reciprocable element.

10. A limit valve for use with a fluid operated cylinder or the like,said valve comprising a body having an elongated chamber and a pluralityof fluid passages communicating with said chamber, at least four of saidpassages being ported to the outside of said body, an elongated valvespool slidably disposed in said chamber for controlling fluid flowthrough said passages, and means including fluid blocking means forselectively interconnecting each of said four ported passages witheither of 8 a predetermined two other of the four ported passages, saidfluid blocking means including fixed lands on the valve spool anddirectional check valves associated with at least three of the fourported passages.

11. The combination of claim 10 wherein at least two of the portedpassages have branches communicating at different points along thelongitudinal length of the chamber, said valve spool being therebyeffective to connect the ported passages in different paired arrangementfor different positions of the valve spool.

l2. The combination of claim 11 wherein an annular recess is provided inthe Wall of the main valve chamber, said recess having a maximumdiameter which exceeds that of the valve spool, at least two fluidpassages having communication with said annular recess, at least one ofsaid two fluid passages being provided with a directional check valve.

13. In a fluid operated power assembly, a fluid motor including areciprocable element, a limit valve having fluid connections with saidmotor for automatically stopping said reciprocable element in apredetermined position intermediate the ends of its maximum stroke, saidlimit valve including a displaceable element for controlling fluid flowbetween the limit valve and the fluid motor, means biasing saiddisplaceable element toward a position of normal rest, and meansdefining fluid passages in said limit valve for cooperation with saiddisplaceahle element in controlling fluid flow, said fluid passagesbeingarranged for cessation of fluid flow between the limit valve andthe fluid motor upon movement of the displaceable element from itsposition of normal rest to a first displaced position and for reversalof fluid flow between the limit valve and the fluid motor upon movementof the displaceable element from its first displaced position to afurther displaced position.

14. The combination of claim 13 plus means interengaging thereciprocable element and the displaceable element for movement of thelatter upon predetermined movement of the former.

15. A fluid operated power assembly comprising, in combination, areversible fluid motor including an oscillating power transmittingelement, a first and a second fluid connection to said motor, a fluidcontrol valve for selectively controlling the oscillations of the powertransmitting element, a fluid pressure connection, a fluid returnconnection, and a first and a second fluid communicating connection tosaid control valve, a limit valve for limiting the movement of the powertransmitting element, a first, a second, a third, and a fourth fluidc0mmunicating connection to said limit valve, fluid conducting meansinterconnecting said first, second, third, and fourth communicatingconnections of the limit valve respectively with the first and secondcommunicating connections of the control valve and the first and secondfluid connections of the motor, means including movable means in-thecontrol valve for selectively intercommunicating the first communicatingconnection therein with either the pressure connection or the returnconnection and the second communicating connection therein with theother of the aforesaid pressure and return connections, and meansincluding movable means in the limit valve for first intercommunicatingthe second with the third and the fourth with the first communicatingconnection therein and thereafter inter-communicating the second withthe fourth and the third with the first communicating connectiontherein.

16. The combination of claim 15 wherein the movable means in the limitvalve includes a displaceable member for changing the intercommunicationbetween connections to the limit valve, and means for interconnectingsaid displaceable member with the power transmitting element forautomatic self-control of the latter.

17. A device of the kind described comprising: a valve chamber; fluidpressure entrance and exit ports each communicating with a differentportion of said chamber,

and fluid exhaust, entrance and exit ports each communicating with adifferent portion of said chamber; control means in said chamber movableto a first position for providing two-way communication between saidpressure ports and between said exhaust ports; a check valvecommunicating with a fluid pressure port and said chamber and a checkvalve communicating with a fluid exhaust port and said chamber; saidcontrol means being movable to a second position for blocking thetwo-way communication; said check valves permitting passage of fluidbetween said pressure ports in one direction only and between saidexhaust ports in one direction only when said control means occupiessaid second position.

18. The device of claim 17 including a second like device wherein thefluid pressure entrance port of the second device is connected to thefirst-mentioned fluid pressure exit port and the fluid exhaust entranceport of the second device is connected to the first-mentioned fluidexhaust exit port and wherein the check valves of the second devicerespectively permit one way fluid flow in directions opposite from thefirst-mentioned check valves when the control means of the second deviceoccupies its second position.

19. A limit valve comprising a body provided with two valve chambers,said chambers being directly intercommunicated by means of at least twoindependent passages providing two-way communication between thechambers, each chamber being further directly communicated with pointsoutside the body by at least two independent passages, an individualvalve spool movably disposed in each valve chamber for controlling fluidflow therethrough, and a directional check valve associated with each ofsaid intercommunicating passages, each of the intercommunicatingpassages between the chambers including a branch passage containing itsdirectional check valve and communicating with one of the chambers at adifferent point than the two-way intercommunieating passage.

References Cited in the file of this patent UNITED STATES PATENTS695,675 Ebel Mar. 18, 1902 2,067,492 Kingsbury Jan. 12, "1937 2,247,141Twyman June 24, 1941 2,447,968 Trotter Aug. 24, 1948 2,475,298 SloaneJuly 5, 1949 2,523,665 Morse Sept. 26, 1950 2,540,467 Williams Feb. 6,1951 2,675,785 Ford Apr. 20, 1954 2,705,971 Dorkins Apr. 12, 19552,710,628 Hodgson June 14, 1955 2,757,641 Meddock Aug. 7, 1956.2,826,258 Livers Mar. 11, 1958 FOREIGN PATENTS 571,979 Germany Mar. 11,1933 552,832 Great Britain Apr. 27, 1943

